Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MRI systems, including MR spectroscopy systems, can be used to study different nuclei, such as 1H, 31P, 13C, 19F, 2H, 29Si, 27Al and 27N and generate images for more than one nuclei. The different nuclei, however, require different resonant frequencies. Various dual-tuned (or multi-tuned) RF coils have been developed for multi-nuclear imaging and provide a single RF coil capable of resonating simultaneously at more than one frequency. Dual-tuned (or multi-tuned) RF coils reduce imaging time and avoid repositioning artifacts that can be caused from changing the RF coil during a scan. The dual tuned RF coils that have been developed include dual-tuned birdcage coils and dual-tuned transverse electromagnetic (TEM) coils. Birdcage coils and TEM coils each have various advantages depending on the resonant frequencies required.
It would be desirable to provide a hybrid RF coil that can be tuned to multiple frequencies and provides the advantages of both a birdcage coil and a TEM coil in a single RF coil structure.